lördag 10 oktober 2015 skrev Josh Dersch <[email protected]>: > > > On Oct 10, 2015, at 4:15 AM, Mattis Lind <[email protected] > <javascript:;>> wrote: > > > > 2015-10-10 5:42 GMT+02:00 Josh Dersch <[email protected] <javascript:;> > <mailto:[email protected] <javascript:;>>>: > > > >> Hey all -- > >> > >> > > First of all I haven't poked around in the PSU of my 11/44 since it > worked > > fine when powered up. But the design of the H7140 is a lot similar to the > > design of the H7104, the PSU of the VAX-11/750 which I worked quite a lot > > with. > > > http://www.datormuseum.se/computers/digital-equipment-corporation/vax-11-750 > < > http://www.datormuseum.se/computers/digital-equipment-corporation/vax-11-750 > > > > > > > >> Once again, I find myself in over my head debugging a power supply, this > >> time an H7140 from a PDP-11/44. Here's the skinny: > >> > >> I examined the supply physically before experimenting and found a > >> capacitor on the Bias/Interface board that was leaking, bursting and > rather > >> burned-out looking (not a great sign) -- this is capacitor C4 in the > >> printsets on Bitsavers ( > >> > http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp11/1144/MP00897_11X44sys_Dec79.pdf > ) > >> Everything else looked OK physically; I replaced the obviously bad > >> capacitor at C4. > >> > >> When power is applied (plugged in, breaker switch flipped to "On") the > >> relay does not click - based on my readings of the manuals this should > >> happen after the bias voltages are up to spec. I measure 308VDC on the > >> lugs on the top of the memory board, so that's at least something > working. > >> Getting to other points to test voltages is a bit more difficult, > >> especially with those high voltages in the way, what a nice design :). > >> > > > > The previous owner said that it happens that the relays get stuck. The > > relay is there to limit the inrush current so when it clicks it bypasses > a > > couple of power resistors. Regardless relay state you will have the full > > voltage over the capacitors. (at least this is the case in the H7104). If > > the relay get stuck then the power resistors get very hot. The relay > drive > > circuit detects that the voltage is high enough then turns on the relay. > > Maybe something in this circuitry is bad. In this case you could force it > > to on but then I think you should power it from a variac > > Thanks. I should have mentioned, but I tested out the relay and it’s not > stuck and seems to be working fine. It definitely looks like the relay > control circuitry is not running properly. > > > > > > > > >> Switching the front panel switch to "Local" (or any other position) has > no > >> effect -- no fans, no LEDs, nothing. I've double-checked all the wiring > >> and everything looks OK. > >> > > > > Have you verified that you have the 12V BIAS voltage? If not you could > > probably generate the 12V BIAS voltage using a lab power supply just to > see > > if the main switcher is working properly. > > > > I used an insulation transformer and a variac and then forced the relay > > drive to on. In the H7104 the same circuitry also enabled the main > switcher > > so unless I did this there were no fun at all. I supplied 12V using a > bench > > supply. It made it possible to check everything was working ok without > any > > high voltages. > > Interesting idea, I’ll look into that. Thanks!
Looking yet another time at the schematic it is evident that a lot of signal is relying on E5 collector beeing low. If the monitor circuit is broken or not high enough voltage in (since you are running it off a variac) you will not have this signal. To test I would ground this and run it with a variac. /Mattis > - Josh > > > > > > > >> > >> Capacitor C4 looks to be involved with the START-UP DRIVE signal > circuitry > >> (which drives the relay) so the behavior I'm seeing makes sense if C4 > died > >> and took a couple of things with it (or if something else died and took > C4 > >> with it). > >> > >> Here's where it gets kind of odd -- I spent some time testing diodes and > >> transistors in the related area near C4 and while doing so I noticed > that > >> there are four diodes (D1-D4) listed on the schematic that are missing > from > >> my board. "Missing" as in someone clipped them out at some point -- > there > >> are just nubs of the leads left. I'm not sure why this would have been > >> done, but there were a number of ECOs applied to this board (a few wires > >> and resistors added) and I don't want to assume that if I just put four > new > >> diodes in that it won't cause other problems. Anyone know if there were > >> other revision levels of the bias/interface board that would have done > away > >> with these diodes? Anyone have an H7140 they can easily crack open to > >> compare? (It's actually relatively easy to get to, if you can get to > the > >> supply...) > >> > >> Thanks as always, > >> Josh > >
